“That which is untenable or beyond the limits of rationality. When associated with existentialism, the absurd refers to there being a lack of any meaning inherent within the real world or in our actions. It gained currency in popular culture via Samuel Beckett’s theatre of the absurd and works by Sartre and Camus. A phrase famously (and erroneously) attributed to Tertullian claimed that faith in an incarnate God was absurd: credo quia absurdum est—’I believe because it is absurd.’
“The actual quotation from Tertullian is: credibile est, quia ineptuin est—’It is credible because it is silly.’ (De carne Christi 5.4). Tertullian is sometimes taken to thereby valorize irrationality, but his thesis was instead that the truth of Christianity was absurd only in relation to Stoic, non-Christian philosophy. If Tertullian is correct, the tenability of Christianity is not contingent upon external, philosophical inspection.”
— A Dictionary of Philosophy of Religion, Charles Taliaferro and Elsa J. Marty, eds., 4.